1. Field of the Invention
The present invention relates to controlled release fertilizer compositions. More, particularly it relates to fertilizer compositions which exhibit release characteristics such that nutrients release from the fertilizer compositions in accordance with a Gaussian release rate curve essentially matching the growth rate pattern of plants to which the fertilizer is applied. The present invention, also, relates to processes for the preparation of such fertilizer compositions. Furthermore, this invention provides methods for treating a plant under field conditions with a fertilizer which demonstrate a nutrient release pattern matching the growth rate pattern of the treated plant.
2. Description of Related Art
Coated (or encapsulated) fertilizers are known to be very effective sources to provide controlled release of nutrients for the feeding of plants. The nutrients are released at controlled rates through the fertilizer's coating resulting in a sustained feeding of plants. As a result, one application of these so-called controlled release fertilizers can provide the necessary nutrients for a plant that would take multiple applications of soluble fertilizers. One type of coated fertilizer in wide use is sulfur coated fertilizer, such as disclosed in U.S. Pat. Nos. 4,042,366; 4,636,242 and 5,405,426. The release of nutrients from sulfur-coated fertilizers occurs by diffusion through imperfections in the sulfur coating and through coating breakdown. The major advantage of the sulfur coated fertilizers is their relatively low cost.
A second type of controlled release fertilizer utilizes solvent applied polymer coatings. The polymeric materials applied are either thermosetting resins or thermoplastics. Examples of solvent applied thermosetting resin coated fertilizers which are currently in use are disclosed in U.S. Pat. Nos. 3,223,518; 4,657,576 and 4,880,455; whereas examples of fertilizers having thermoplastic coatings are disclosed in U.S. Pat. No. 4,019,890. Another type of encapsulated fertilizer that exhibits good controlled release properties is latex coated granular fertilizers such as those disclosed in U.S. Pat. Nos. 4,549,897 and 5,186,732. Both solvent and latex applied polymer coated fertilizers offer important benefits over sulfur-coated products in regard to consistency of release rates. The majority of nutrient release is by diffusion through pores in the polymer coating, rather than release through coating imperfections.
The presence of a polymeric coating on controlled release fertilizers allows for a rather uniform and consistent nutrient release, provided that the barrier properties of the polymer are sufficient. However, in general these release fertilizers, after application to a plant, exhibit an initial rapid release of nutrients, followed by a decreasing rate of release during a succeeding period and a constant release at a sufficient level thereafter. Finally, the fertilizer granules become exhausted resulting in a further decrease in release rate. In general, the accumulated nutrient release curves can be characterized mathematically by smooth quadratic (convex) curves. Such quadratic release curves have been transformed into release rate curves showing a continuous decrease in nutrient release.
In practice, this implies that such controlled release fertilizers do not release nutrients in accordance with the specific nutrient requirements for growth of a plant treated with the fertilizer since data available from universities and trial stations indicates that numerous plant growth rate patterns resemble Gaussian type curves as illustrated herein in FIG. 1 which demonstrate that plant growth rate follows a Gaussian pattern over time increasing to a maximum level and then declining therefrom. Accordingly, the desirability of providing fertilizers which exhibit a Gaussian release rate characteristics that match the growth rate pattern of a plant species or a plant variety treated with the fertilizer has been recognized heretofore. Such a release pattern implies that the maximum nutrient release rate from the fertilizer composition will coincide with the time the growth of the plant is highest (i.e., when the demand of the plant is the highest).
By converting Gaussian release rate curves into accumulated release curves, S-type (or sigmoid) curves are obtained. Heretofore, controlled release fertilizers which exhibit release of nutrients according to such S-type curve have required the presence of multiple coatings and/or the incorporation of specific additives in the coating. For example, the text Controlled Release Fertilizers With Polyolefin Resin Coating, edited by Sadao Shoji and Ambrosio T. Gandeza, published by Konno Printing Co., Ltd., Sendai, Japan, 1992, at page 30, teaches that multi-layered polyolefin-coated controlled release fertilizers exhibit S-type or sigmoid release curves as a result of the incorporation of specific chemicals in the coating. Products having a first and a second coating showing an S-type release curve are disclosed in U.S. Pat. No. 5,652,196.
However, no controlled release fertilizers comprising a single coating layer applied over a nutrient fertilizer core granule have been known heretofore which release nutrients according to an S-type or sigmoid curve so as to provide nutients to plants treated with the fertilizers in accordance with the specific nutrient requirements of the plants.